I have noticed a few yellow-faced blue tits, one yellow-browed redpoll, and a yellow-browed goldfinch in the garden. Thinking that the most likely explanation was pollen rather than these being new species, I turned to Google. It turns out that blue tits are known to visit goat willow and grey willow to feed on nectar from catkins, as shown in this article published in Bird Study in 1985. I also found a YouTube video of redpolls feeding on willow catkins which you can see here. This behaviour is also reported in blackcaps and whitethroats.
Then this morning going to work in Gosforth Park Nature Reserve I saw blue tits feeding on the pussy willow. It was too far away for a photo (and I didn't have a camera!).